His team has worked through more than 130 people of interest, narrowing them to a list of 12 men and women involved in either planning or killing Ms Tolley or assisting her killers after the attack.

This has allowed police to piece together a "full picture" of what had happened but more evidence was still needed to lay charges, Det Sen Sgt Barnett said.

"Over the course of the investigation we have spoken to at least 15 people who have direct knowledge of who planned the attack, committed the murder and actively helped to cover up the offenders' involvement."

But up until this point, this group had refused to co-operate with the investigation, he said.

"We are seeing this attitude slowly changing with a number of people starting to come forward with valuable information.

"However, we still need more of the people, who have spoken with the offenders, to contact us."

It comes after Lois Tolley's mother, Cathrine, and more than 50 friends gathered in an Upper Hutt park on October 10 this year to remember her.